1. Field of Invention
This invention relates to imaging and, in particular, to an apparatus and method for multi-spectral dual balanced imaging.
2. Description of Related Art
Multi-spectral imaging systems produce images, such as images of objects and scenes, at more than one band or range of wavelengths of the electromagnetic spectrum. Some multi-spectral imaging systems can produce first and second simultaneously captured images at first and second spectrally distinct ranges of infrared wavelengths, respectively, and can display a differential image representing the difference between the first and second images. Such multi-spectral imaging systems are useable to display a representation of a scene or object, such as a gas plume, that is associated with electromagnetic radiation in the first, but not second, range of infrared wavelengths.
However, such multi-spectral imaging systems produce significant variations in relative intensity between the first and second images at respective pixel coordinates of the first and second images in response to a uniform incident light; produce ghost images due to the Narcissus effect of detecting as incident light electromagnetic radiation produced within the system itself; produce differential images with insufficient contrast between the first and second images, particularly in the absence of a high intensity incident light; are not suitable for imaging while the system undergoes movement; cannot produce differential images having high contrast in the case of wide ranges of infrared wavelengths nor in the case of multiple selectable ranges of infrared wavelengths; cannot produce images suitable for determining a flow rate associated with the gas plume; are too heavy for portable use; and include optical distortions, such as differences in optical path length of the respective optical paths within the system associated with the first and second images, lens aberrations such as barrel and trapezoidal lens aberrations, optical distortions resulting from mechanical vibrations within the system and parallax distortions in the optical paths, that impede pixel registration of the respective pixel coordinates.
In particular, such multi-spectral imaging systems include a pair of detectors that may be calibrated using a two-point Non Uniformity Correction (NUC) type calibration, which involves adjusting gain and offset of pixel elements of each detector to produce uniform output in the presence of uniform incident light associated with two different temperatures. However, such two-point calibration technique cannot calibrate for non-linearity between wavelength and gain and cannot calibrate for non-linearity between wavelength and offset. Thus, such multi-spectral imaging systems are subject to a non-linearity affecting comparisons of the pair of images that cannot be corrected using the two-point NUC type calibration.